1. IEEE/NSS Oct 22, 2008 1 Electron Counting and Energy Resolution Study from X-ray conversion in Argon Mixtures with an InGrid-TimePix detector. D. ATTIÉ 1), M. CAMPBELL 2 ), M. CHEFDEVILLE 3), P. COLAS 1), E. DELAGNES 1), K. FUJII 4), Y.GIOMATARIS 1), H. VAN DER GRAAF 3), X. LLOPART 2), J. TIMMERMANS 3), J. VISSCHERS 3) 1) Irfu,CEA Saclay; 2) CERN ; 3) Nikhef; 4) KEK

2. IEEE/NSS Oct 22, 20082 Gain fluctuations An old problem : Wejsman 1949, Legler 1955, 1961, Riegler 2003,… With a renewed interest: space resolution of an MPGD tracker (gain fluctuations lower by a factor (1+  )/(2+  ) the number of effective electrons, D. Arrogancia et al., accepted in NIM A) threshold Efficiency for single-electron detection And with new means of investigation : Microbulk, InGrid + TimePix G/ Avalanche size distribution

3. IEEE/NSS Oct 22, 2008 3 Gain fluctuations Though there is no clear justification for this, we use Polya to parameterize the gain distribution. For  =0, the distribution is an exponential (Furry model) For  =1, the analytical integrations (average, efficiency) are easy Alternative convention is parameter m=1+ 

4. IEEE/NSS Oct 22, 20084 Micromegas + TimePix TimePix READOUT MICROMESH DRIFT DRIFTSPACE E D ~ 1 kV/cm E D ~ 1 kV/cm E A ~ 80 kV/cm InGrid (Nikhef-Twente) 55 Fe Cr filter Micromegas detector 5.9 keV Xrays giving 220 elec. in argon with rms sqrt(F/220) Peak width: contribution from primary (Fano) fluctuations and gain fluctuations (assuming high detection efficiency) Sqrt((F+B)/N)

5. IEEE/NSS Oct 22, 20085 Chromium K-edge (Center for X-Ray Optics) Microbulk detector KEK, january 2007

6. IEEE/NSS Oct 22, 20086 Result : 5.6% r.m.s. resolution (Broken record) Noise very small thanks to adequate filter on the mesh

7. IEEE/NSS Oct 22, 20087 5% rms resolution InGrid measurements

8. IEEE/NSS Oct 22, 20088

9. 9  = 2 For F=0.20 Resolution (rms) Theta parameter

10. IEEE/NSS Oct 22, 200810 In the gain fluctuations, the avalanche statistics and the effect of the field configuration in the hole cannot be disentangled. A full simulation shows how the resolution depends on the detector geometry.

11. IEEE/NSS Oct 22, 200811 TimePix chip Idea : take a medical imaging chip (Medipix 2), add a clock to each pixel, replace ‘grey levels’ by ‘clock ticks’ (Michael Campbell, Xavi Lloppart, CERN) 65000 pixels, 14-bit counter, 100 MHz tunable clock frequency -> more voxels than the ALEPH TPC, but tiny! 55  m Pixel 14111  m 16120  m 14080  m (pixel array) 11 2233 44 55 55 μ m μ m Preamp/shaper THL disc. Configuration latches Interface Counter Synchronization Logic

12. IEEE/NSS Oct 22, 200812 See electrons from an X- ray conversion one by one and count them, study their fluctuations (Nikhef-Saclay)

13. IEEE/NSS Oct 22, 200813 Study efficiency vs gain SiProt on chips: spreads the charge over 2-3 pads: count clusters Use the rms size of the x-ray spot to select contained events Use the time distribution to remove noise

14. IEEE/NSS Oct 22, 200814 Measured spectra at -330 V Timepix #1 Timepix #1 Timepix #2 Timepix #2 5.9 and 6.5 keV escape events (event ratio ~ 7:1) 5.9 and 6.5 keV escape events (event ratio ~ 50:1)

15. IEEE/NSS Oct 22, 200815 Peak position and grid voltage Asymptotic value of N d gives the number of collected electrons N c Polya fit works very well where exponential one (not shown) fails! N c = 115 e - N c = 102 e - Compatible with the smaller hole diameter of InGrid #2 Contribution from collection efficiency to peak width now known

16. IEEE/NSS Oct 22, 200816 W and F in Ar/iso 95/5 at 2.9 keV Assume full collection efficiency of detector #1 N p = N c = 115 ± 2 e- W = 25.2 ± 0.5 eV Peak width measured with detector #2 corrected for detection and collection eff. (87 %) RMS(N p ) ~ 4.3 % F = 0.21 ± 0.06 W = 25.0 ± 0.6 eV F = 0.250 ± 0.010 Ar/iso 20/80 – 1253 eV X-rays from Pansky. et al. J. Appl. Phys. 79 (1996) 8892 Extrapolation to 5.9 keV photo-peak straightforward N p = 230 ± 4 e- Consistent with Pansky et al. 25.0 ± 0.6 eV Consistent with measured values and theoretical estimate 0.17 for pure Ar

17. IEEE/NSS Oct 22, 200817 Conclusions New ‘almost perfect’ detectors give gain fluctuations wich can be parametrized by polya with  ~ 1 or 2. New ‘almost perfect’ detectors give gain fluctuations wich can be parametrized by polya with  ~ 1 or 2. Fano fluctuations are now accessible by electron counting. Fano fluctuations are now accessible by electron counting. Best resolution understood as sqrt((F+B)/N ), with F=0.2 and B=0.6 for Micromegas Best resolution understood as sqrt((F+B)/N ), with F=0.2 and B=0.6 for Micromegas Most of the work presented here was carried out by Max Chefdeville for his PhD thesis.